Gestational diabetes mellitus (GDM) is linked with several acute maternal risks, including preeclampsia, caesarean sections, induced deliveries, and pre-term labor, and long-term risks of type 2 diabetes, metabolic syndrome, and cardiovascular disease. Women with GDM are more likely to display features of insulin resistance - a strong precursor to type 2 diabetes and cardiovascular disease (CVD). Intrauterine exposure to GDM raises neonatal health risks, including fetal demise, macrosomia, and birth trauma and long-term risk of metabolic disease. GDM recurrence is also common, affecting 40- 73% of women overall, and augments associated maternal/fetal health risks. Preliminary data from epidemiologic and bariatric surgery studies suggest that reducing body weight before pregnancy can prevent development of GDM and associated maternal/fetal complications. Our preliminary research suggested that it was feasible to recruit women before pregnancy and promote significant weight loss prior to conception. However, a full scale trial to test the effects of maternal lifestyle intervention before pregnancy to reduce body weight and prevent GDM recurrence has never been conducted. The principal aim of this study is to determine whether a lifestyle intervention to reduce body weight before pregnancy can reduce GDM recurrence and improve biomarkers of insulin resistance and cardiovascular disease. This two-site trial proposes to recruit 350 overweight/obese women with previous histories of GDM who plan to have another pregnancy in the next 1-2 years. Women will be randomized within site to a 16-week comprehensive pre-pregnancy lifestyle weight loss intervention (with ongoing treatment until conception) or an educational control group. Assessments will occur before pregnancy (at study entry, after 4 months, and at brief quarterly visits until conception), during pregnancy (at 26 weeks gestation) and at delivery at 6 weeks postpartum. Secondary aims will examine treatment-related reductions in prenatal and perinatal complications and changes in pre- pregnancy weight, eating, and activity. If found effective, the proposed lifestyle interventio could shift current treatment practices towards the interconception period and provide an evidence based immediately useable therapy targeted to optimize reproductive outcomes and prevent GDM and associated chronic conditions in the more than 200,000 US women diagnosed with GDM each year. The study is also innovative, as there have been no randomized controlled trials evaluating the effects of a lifestyle intervention delivered before pregnancy to prevent GDM. With a focus on prevention, the proposed study will directly challenge the current conventional approach to GDM - which focuses on early identification and treatment during pregnancy - and shift efforts towards the preconception period.